This invention relates to radars and more particularly to a rotary joint applicable for all frequencies and to millimeter wavelengths, in particular.
Rotary joints provide a continuous microwave transmission path between rotating and stationary sections of a mechanically scanned antenna system. They must operate over the scan range of the radar system with minimum distortion of the microwave signal. To do this, the voltage standing wave ratio (VSWR) (reflection) and insertion loss of a rotary joint needs to be minimized and have minimal variation with rotation over the desired frequency band.
Microwave energy propagates in waveguide only in particular modes FIGS. 1a and 1b. In rectangular waveguide, used for transmission paths in most radar systems, the energy propagates in the dominant TE.sub.10 (transverse electric wave). For rotary joints, this energy must first be converted to a circularly symmetric mode and waveguide (circular tube or coaxial line). A circularly symmetric mode implies that the orientation of the E (electric) and H (magnetic) field patterns in the waveguide make the modes independent of rotation. In the circular tube, a break between rotating and stationary parts of the rotary joint can be made with a small gap RF choke providing electrical continuity at the break. At the output of the rotary joint a conversion back to the TE.sub.10 in rectangular waveguide is needed. Those persons skilled in the art desiring more information about a rotary joint with a small gap RF choke are referred to "Radiation Laboratory Series #9--Microwave Transmission Circuits", George L. Ragan, pp. 193-199.
In the past (FIG. 2), rotary joints have used a right angle transition from the TE.sub.10 mode in rectangular waveguide to the TM.sub.01 mode in circular waveguide. A circular hole has been cut in the broad wall of the rectangular waveguide the same diameter as the desired circular waveguide and the two waveguides are attached. The size of the circular waveguide is chosen to propagate the TM.sub.01 mode at the design frequency but small enough to be in the non-propagating region of any higher order modes. Shorting stubs are inserted in the open ends of the rectangular waveguides.
The shape and position of these stub "tunes" the rotary joint to operate in the desired frequency band. The higher the frequency the smaller the parts become. For example, rectangular waveguide used in the 12-18 GHz range has a width of 0.622 inches wide compared to 0.100 inches for waveguide used at 94 GHz. Surface finish inside the waveguide becomes more critical at higher frequencies since the wavelength of the energy becomes proportionally smaller. The rectangular to circular waveguide right angle transition would be difficult and expensive to build at millimeter wavelengths.
The same fabrication techniques and design principles used at lower frequencies can not be used to build an inexpensive millimeter wave rotary joint. Most millimeter wave components are made out of expensive coin-silver or plated materials which are necessary to keep losses low at these high frequencies. Intricate components can be made using electro-forming, casting, or other similar techniques, but all are expensive processes and some final machining operations would still be necessary for rotary joint parts.
In addition to the mechanically scanned antenna, conical scan or twist reflector type antenna systems have been studied for radar systems operating at millimeter wave frequencies (above 40 GHz). These systems are less efficient in performance and are more costly.